Here we demonstrate high fidelity control of ferroelectric domain structures in a z-cut LN nano-photonic waveguide, which enables first order quasi-phase matching for near IR to blue second harmonic generation with a poling period (\(\sim 2\,\mathrm {m}\) ) and 40% duty cycle. By exploiting the strongest nonlinear co-efficient \(\mathrm {d_{33}}\) , we show bright, efficient blue light generation with a conversion efficiency consistent with theoretical limit. Our advance of operating thin film LN to the lower wavelength edge of its transparency window makes this nonlinear material platform particularly attractive for visible photonic applications in classical and quantum regimes. To highlight this utility, we generate blue SH at 435.5 nm which targets the transition wavelength for Ytterbium-ion clocks with an on-chip efficiency 2900\(\pm \) 400%/W/cm\(^2\) . To the best of our knowledge, this is a record efficiency for blue second-harmonic generation in an integrated thin film material platform. Besides SH generation, we also assess the linear loss of LN in this new operating wavelength regime, which is found to be \(3.9\pm 0.5\,\) dB/cm. Unlike resonant PPLN doublers which display strong photo-refractive effect [1]}, [2]}, [3]}, [4]}, we show stable device operation even in deep visible band. Temperature control of the waveguides further allows the tuning of phase matching window over 3 nm wavelength span (6 nm at fundamental wavelength) with a 0.068 \(\mathrm {nm/^oC}\)  tuning rate, ensuring their practical applications that require precision alignment with specific atomic transitions.
